If you are traveling in the Western Hemisphere, (Canada, Mexico, Caribbean, Central and South America):
ALL persons are required to present a valid passport, Air NEXUS card, or U.S. Coast Guard Merchant Mariner Document.
Oral declarations are no longer accepted. Travelers will need to present a valid government issued ID and a birth certificate or naturalization certificate.
All U.S. citizens who are 16 and older are required to present a valid passport or other alternative documents as determined by the Department of Homeland Security. U.S. and Canadian citizens who are 15 years old or younger will still be allowed to travel with just a copy of their birth certificate, as will teens between the age of 16-18 if they are part of an adult supervised school, religious, cultural or athletic group.
Lawful Permanent Residents (Green card holder's) do NOT need a passport to enter the United States.
Those who drive a vehicle into Mexico are required to purchase Mexican Auto Insurance (it is the law in Mexico just like here in the United States).
Below are a few reputable websites to purchase Mexican auto insurance online.
More links will be added in the near future.